ROVs and, at times, AUVs require tools when operating underwater, such as to work on pipelines, wellheads, and other structures while underwater. Most often, these tools are taken down or otherwise supplied one at a time, interactively, by returning the ROV or AUV to a surface location to allow manual exchange of the tools.
However, in certain situations, once on site at a remote location (potentially under ice), an ROV or AUV may have to stay deployed underwater for a long time, e.g. up to six months or more, and operate without any maintenance or direct operator intervention. This means that there is no one to manually change out ROV tools as is the normal oilfield practice.